The product of the present invention has utility as a ceramic electrolyte for sodium-sulfur batteries. These batteries have high energy and power densities that make them useful in electric vehicle propulsion and energy storage systems for load leveling applications in electric generating plants. Other commercial applications of beta-aluminas include liquid sodium purification and thermoelectric power generation.
The term beta-aluminas as used herein is meant to include both beta and beta"-alumina. These could occur together or alone depending on the relative concentration of sodium and aluminum.
Typical literature that describes the state of the art includes: "Formation of Continuous Beta-Alumina Films and Coatings at Low Temperatures", B. E. Yoldas et al, Ceramic Bulletin, Vol. 59, No. 6, pp. 640-642 (1980) that describes a method that allows a sintering temperature of 1200.degree. C. "Recent Progress in the Development of Beta-Alumina for the Sodium-Sulphur Battery", G. J. May et al, Electrochemica Acta, Vol. 24, pp. 755-763 (1979) which describes progress and problems with production of beta-aluminas. "Sintering of Beta-Alumina Powders Obtained by Sol-Gel Process", Preliminary Studies, A. Deptula et al, Proceedings of the 5th International Round Table Conference on Sintering, Material Science Monographs, Vol. 14, pp. 219-226 (1982) which is a study of optimum conditions for obtaining beta"-alumina.
U.S. patents that are typical of art in this area include the below.
U.S. Pat. No. 4,244,986, which describes a process for forming sodium beta-aluminas by forming an agglomerate free hydrolized sol with an acid peptizing Na(OR) and Al(OR.sub.3) alkoxide compounds in such a manner that a slurry of surface active polymers containing Na, Al, OR, and OH groups are formed. The peptizing acid is absorbed on the polymer surface. Initially, excess Na.sub.2 O is required because of Na.sub.2 O loss during subsequent heat treatment. Gelation is avoided by keeping the aluminum concentration of the sol below 2.5 equivalent percent. The sol is heated at 1200.degree. C. to 1400.degree. C. to form crystalline sodium beta-aluminas.
U.S. Pat. No. 4,208,475 illustrates a method for making an ion conductive ceramic by reacting partially hydrolized sodium and aluminum alkoxides together with heat stabilizers such as lithium, magnesium and potassium alkoxides, oxides or carbonates. Excess sodium is required and complete crystallization takes place only above 1200.degree. C.
U.S. Pat. No. 4,083,919 describes a method of producing beta-aluminum at lower temperatures. Beta-alumina is precipitated as a gel and an amorphous material is produced by heating to about 900.degree. C. Further heating and pressing at elevated temperatures, about 1200.degree. C. and about 4000 psi, is needed to obtain a crystalline ceramic product.
Other methods for preparing beta-aluminas are known in the literature and patent art. One of the most common methods involves calcination of mechanically mixed alpha-alumina and dopant salts followed by ball milling to produce sinterable powder. Other preparation methods include spray drying of an aqueous slurry of boehmite or alpha-alumina and soluable alkalies, co-precipitation of complex oxylates of the constituent elements, and spray freezing or freeze drying of an aqueous solution of the soluble salts of the constituent elements.
The crystallization behavior of the powders synthesized by the above procedures show three types of crystallization paths. Because chemical inhomogeneity of the powders is inherent in all the above processes, it is difficult to control the microstructure of the end product. This microstructure is of great importance for fabricating high quality electrolyte bodies.
It is an object of this invention to overcome the difficulties of the prior art by providing a product of high homogeneity capable of being made into a crystalline beta-alumina at lower temperatures than heretofore possible.